The present invention relates to a rotary stirring device for treating a molten metal and to metal treatment equipment comprising such a device.
It is well known that molten metal, in particular non-ferrous molten metals such as aluminium alloys, must be treated before casting, typically by one or more of the following processes in order to:
i) Degas—The presence of dissolved gas in molten metal can introduce defects in the solidified product and may reduce its mechanical properties. For example, defects are introduced in castings and wrought products manufactured from aluminium or its alloys. Hydrogen has a high solubility in liquid aluminium which increases with melt temperature, but the solubility in solid aluminium is very low, so that as the aluminium solidifies, hydrogen gas is expelled causing gas pores in the casting. The rate of solidification influences the amount and size of the bubbles and in certain applications the pinhole porosity may seriously affect the mechanical strength and the pressure tightness of the metal casting. Gas may also diffuse into voids and discontinuities (e.g. oxide inclusions) which can result in blister formation during the production of aluminium alloy plate, sheet and strip.ii) Grain refine—Mechanical properties of the casting can be improved by controlling the grain size of the solidifying metal. The grain size of a cast alloy is dependent on the number of nuclei present in the liquid metal as it begins to solidify and on the rate of cooling. A faster cooling rate generally promotes a smaller grain size and additions of certain elements to the melt can provide nuclei for grain growth.iii) Modify—The microstructure and properties of alloys can be improved by the addition of small quantities of certain ‘modifying’ elements such as sodium or strontium. Modification increases hot tear resistance and improves alloy feeding characteristics, decreasing shrinkage porosity.iv) Cleaning and Alkali Removal—Certain levels of alkali elements may have adverse effects on alloy properties and therefore they need to be removed/reduced. The presence of calcium in casting alloys interferes with other processes such as modification, whereas sodium has a deleterious effect on the ductile properties of wrought aluminium alloys. The presence of non metallic inclusions such as oxides, carbides and borides entrained in the solidified metal adversely affects the physical and mechanical properties of the metal, and they therefore need to be removed.
These actions may be carried out individually or together by a variety of methods and equipment. One approach for adding metal treatment substances is to add them directly to the molten metal as powder, granules or encapsulated in a (aluminium or copper) metal can, whilst mechanically stirring the molten metal to ensure effective distribution throughout the melt. Particulate metal treatment agents may also be introduced by the use of a lance with an open discharge placed below the surface of the molten metal. Powdered or granulated additives are then injected down the lance under pressure using a carrier gas. The lance is typically a hollow tube of graphite or silicon carbide with a thin walled steel insert tube through which the additives and gas are passed.
Degassing of molten metal is typically conducted using a rotary degassing unit (“RDU”) by flushing the molten metal with fine bubbles of a dry inert gas such as chlorine, argon, nitrogen or a mixture thereof. Commonly this is carried out using a hollow shaft to which a rotor is attached. In use the shaft and rotor are rotated and gas is passed down the shaft and dispersed into the molten metal via the rotor. The use of a rotor rather than a lance is more efficient since it generates a large number of very fine bubbles at the base of the melt. These bubbles rise through the melt and hydrogen diffuses into them before being ejected into the atmosphere when the bubbles reach the surface. The rising bubbles also collect inclusions and carry them to the top of the melt where they can be skimmed off.
In addition to introducing gas to remove hydrogen (and oxide inclusions), the rotary degassing unit may also be used to inject metal treatment substances (also known as treatment agents) along with the gas via the shaft into the melt. This method of injection has similar drawbacks to that of lance injection, in that the metal treatment substances are prone to partial melting in the shaft causing blockages, particularly when using powdered material. The introduction and use of granular fluxes alleviated many of the difficulties, as did changes in equipment design.
One such example of equipment for both degassing and metal treatment is the Metal Treatment Station (MTS) developed and sold under the same trade name by Foseco. The first (“MTS”) unit included an accurate dosing unit to allow treatment substances to be added via the shaft and then distributed via the rotor throughout the melt.
As an alternative to using the shaft to introduce the metal treatment agents, later equipment (the “MTS 1500” unit sold by Foseco) adds the treatment substances directly to the melt surface rather than via the shaft and rotor. In the MTS 1500, rotation of the rotor and shaft, within certain parameters, is used to form a vortex around the shaft. The metal treatment agents are then added into the vortex and readily dispersed throughout the melt. Any turbulence in the melt will lead to the introduction of air, and subsequently lead to the formation of oxides in the metal. Therefore the vortex is only employed for a short part of the treatment cycle and once the mixing stage is complete, it is stopped (e.g. by application of a baffle plate). An efficient rotor will create a vortex and disperse the treatments agents as quickly as possible in order to keep the turbulence in the melt to a minimum. Degassing and removal of the reaction products from the melt is then carried out. The intense mixing action of the initial vortex followed by the quiescent part of the cycle (e.g. after the baffle plate has been lowered) leads to efficient use of the treatment agents and optimum melt quality.
An example of a rotary device for use in a rotary degassing unit either with or without an additional process stage such as in a Metal Treatment Station is the “XSR rotor” (prior art rotor 1) described in WO2004/057045 (the entirety of which disclosure is included herein by reference) and shown in FIG. 1. The rotary device 2 comprises a shaft 4 having a bore 4a therethrough connected at one end to a rotor 6 via a tubular connection piece (not shown). The rotor 6 is generally disc-shaped and comprises an annular upper part (roof 8) and spaced therefrom an annular lower part (base 10). An open chamber 12 is provided centrally in the base 10 and extends upwardly to the roof 8. The roof 8 and base 10 are connected by four dividers 14 which extend outwardly from the periphery of the chamber 12 to the periphery of the rotor 6. A compartment 16 is defined between each pair of adjacent dividers 14, the roof 8 and the base 10. The peripheral edge 8a of the roof 8 is provided with a plurality (eight in this embodiment) of part-circular cut-outs 18. Each cut-out 18 serves as a second outlet for its respective compartment 16.
A further prior art rotor is the rotor sold primarily for degassing only by Vesuvius under the trade name Diaman™ (prior art rotor 2) and shown in plan view in FIG. 2. It is generally disc-shaped and comprises four radial bores 22 equiangularly spaced around the rotor 20. Each bore 22 extends from the inner surface of the rotor 20 to its peripheral surface 20a thereby providing an outlet 24 for the gas. The rotor has four cut-outs 26 that extend inwardly from the peripheral surface 20a of the rotor. Each cut-out 26 is located at an outlet 24 and extends downwardly for the entire depth of the rotor 20. There is no chamber for the mixing of gas and molten metal. In use the rotor is attached to a hollow shaft (not shown).
U.S. Pat. No. 6,056,803 discloses an injector for injecting gas into molten metal. The injector consists of a smooth faced rotor attached to the bottom end of a cylindrical shaft. The rotor is in the form of an upright lower cylindrical portion and an upper conical portion. The lower cylindrical portion is provided with a centrally-located cavity from which several passages extend radially. Gas passageways introduce gas into the passages but lack direct communication with the cavity.
DE 103 01 561 discloses a rotor head having a truncated cone shape with a central bore. The side of the rotor head is contoured by the presence of lateral grooves and the underside comprises radially extending channels.
U.S. Pat. No. 5,160,593 discloses a multiple-vaned impeller head that is adapted for mounting on a hollow impeller shaft and is used to treat molten metal. The impeller head has a hub with a central axial bore and a number of vanes are fixed to and extend beyond the hub. The vanes create turbulence for enhancing liquid and gas interphase interaction.